The present disclosure is related to thermoacoustic devices, and more specifically to a thermoacoustic device employing an acoustic energy converter and electrical impedance network in place of selected portions of an acoustic impedance network.
The Stirling cycle is a well-known 4-part thermodynamic process, typically operating on a gas, to produce work, or conversely to effect heating or refrigeration. The 4 parts are: isothermal expansion, isochoric heat extraction, isothermal compression, and isochoric heat addition. The process is closed, in that the gas remains within the system at all times during the cycle.
One device that takes advantage of the Stirling cycle is the Stirling refrigerator. A typical Stirling refrigerator has one or more mechanical pistons, which control the heating/expansion and cooling/contraction of a contained gas as part of the Stirling cycle. Expansion of the gas as part of the Stirling cycle serves to cool a load. An element, typically called a regenerative heat exchanger or regenerator, increases the refrigerator's thermal efficiency. Devices of this type are often complex, involve seals, pistons, etc., and require regular maintenance.
Related types of refrigeration devices are thermoacoustic refrigerators. These devices share some fundamental physical properties with Stirling refrigerators, namely a contained gas which approximates a Stirling cycle. However, a thermoacoustic refrigerator differs from a Stirling refrigerator in that acoustic energy drives a temperature differential for extracting heat from the load. Unlike conventional Stirling refrigerators, the gas within a thermoacoustic refrigerator does not travel significantly within the body structure. Rather, the pressure wave propagates through the gas and the Stirling cycle takes place locally inside the regenerator.
Thermoacoustic refrigerators may operate with either substantially standing wave or traveling wave acoustic phasing in the regenerator. Standing-wave devices are known to be less efficient than traveling-wave devices.
FIG. 6 is a cross-sectional representation of one example 30 of known traveling-wave thermoacoustic refrigerator designs, known as an orifice pulse-tube refrigerator. As is typical, device 30 comprises a hollow, tubular, body structure 32 having a regenerator 34 located therein. Regenerator 34 is often simply a metal mesh or matrix. Regenerator 34 is proximate a first heat exchanger 36, generally a “hot” or “ambient” exchanger often at room temperature, at a first end thereof and a second heat exchanger 38, generally a “cold” exchanger, at the opposite end thereof. A third heat exchanger 39, generally at hot or ambient temperature, is typically present. An acoustic impedance network 40 is provided at one end of body structure 32. A motor and piston 42 is provided at the end of body structure 32 opposite acoustic impedance network 40. A pressurized gas is sealed within body structure 32. Acoustic energy in the form of a pressure wave generated by motor and piston 42 subjects the gas to periodic compression and expansion within regenerator 34. Under favorable conditions, the gas effectively undergoes an approximate Stirling cycle in the regenerator. This induces a temperature differential across the regenerator, i.e., between the hot and cold heat exchangers. Heat transfer may then be obtained between the gas and the heat exchangers, such that heat may be removed from the “cold” heat exchanger.
The acoustic impedance network 40 sets the relative phasing between the pressure and velocity waves so that the gas in contact with the regenerator approximates a Stirling cycle. This creates the thermal gradient between the “cold” and “hot” heat exchangers. However, in a pulse-tube refrigerator, no power is recovered in the gas expansion portion of the cycle. Therefore, the theoretical maximum efficiency of typical pulse-tube refrigerators is limited in comparison with that of Stirling refrigerators.
There are numerous other examples of Stirling and thermoacoustic refrigerators known in the art. U.S. Pat. No. 7,263,837 to Smith, U.S. Pat. No. 7,240,495 to Symko et al., and U.S. Pat. No. 6,804,967 also to Symko et al. illustrate several examples. Each of these U.S. patents is incorporated herein by reference. However, each of these examples presents its own set of disadvantages. One disadvantage of certain prior art devices is the dissipation of power in the acoustic impedance network, limiting their maximum theoretical efficiency. As the relative amount of power lost is greater with higher cold temperatures, this has inhibited the usefulness of thermoacoustic refrigerators for near-room-temperature applications. Another disadvantage of some prior art devices is the relatively large size of the acoustic impedance network. The size is a disadvantage for many applications, where a compact device is required.